One process in the production of coated conductors (superconductive tapes or films) has been referred to as a thick film process. In the deposition of thick films for such coated conductors where the thickness of the superconductive layer is generally at least one micron in thickness, the use of polycrystalline substrates, e.g., polycrystalline metal substrates has been preferred. Buffer layers play an important role in the production of high critical current density superconducting films on polycrystalline metal substrates. Suitable buffer layers can provide the necessary structural template for subsequently deposited superconducting layers. For example, a yttria-stabilized zirconia (YSZ) buffer layer deposited by ion beam assisted deposition (IBAD) has been described by both Iijima et al., U.S. Pat. No. 5,650,378, and Russo et al., U.S. Pat. No. 5,432,151. Similarly, Arendt et al., U.S. Pat. No. 5,872,080 described a coated conductor having the structure YBCO/Y2O3/YSZ/Al2O3/Ni alloy with a high critical current density (Jc) of about 1×106 A/cm2 and a high transport critical current (Ic) of from about 100 to about 200 A/cm. While this current was satisfactory, the deposition of the YSZ layer was considered too slow for commercial production.
U.S. Pat. No. 6,190,752, Do et al., described thin films of a material having a rock salt-like structure deposited by IBAD upon amorphous substrate surfaces. Their preferred material having a rock salt-like structure was magnesium oxide (MgO). In comparison to the deposition of YSZ, MgO can be rapidly deposited through an IBAD process. The structures of U.S. Pat. No. 6,190,752 included, e.g., YBCO/Y2O3/YSZ/MgO/MgO(IBAD)/Si3N4/Ni alloy with a NiO layer in between the YSZ layer and the MgO layer in most instances. Despite the improvement in processing speeds, their structures had Ic's of only about 50 to about 75 A/cm. In addition, at the elevated processing temperatures needed to form the superconductive layer, the silicon nitride layer can react with other materials in the system.
WO 02/47119, Arendt et al., described substrate structures including a layer of an inert oxide material such as aluminum oxide on the surface of the polycrystalline metallic substrate, a layer of an amorphous oxide or oxynitride material such as yttrium oxide or aluminum oxynitride on the inert oxide material layer, and, a layer of an oriented cubic oxide material having a rock-salt-like structure such as magnesium oxide upon the amorphous oxide or oxynitride material layer. One exemplary structure described in WO 02/47119 included, e.g., YBCO/CeO2/YSZ/MgO(IBAD)/Y2O3/Al2O3/Ni alloy. The critical current density (Jc) was measured as 1.4×106 A/cm2 using a standard four-point measurement. The projected transport critical current (Ic) was 210 Amperes across a sample 1 cm wide. WO 02/47119 also described a structure wherein a single layer of erbium oxide (Er2O3) was between the metal substrate and the IBAD-MgO layer.
Despite the prior results shown by Do et al. and Arendt et al., continued improvements in the structure and resultant properties of coated conductors have been desired. Specifically, the present IBAD MgO systems have some drawbacks that detract from their viability as a template layer for long length processing of coated conductors. The two biggest limitations are: (1) the general use of two layers, one layer used as a diffusion barrier and the second used as a nucleation layer; and (2) the need to deposit IBAD MgO films on very smooth substrates(<2 nm rms). After extensive and careful investigation, improvements have now been found in the composite substrate architecture for subsequent preparation of superconducting films thereon. In particular, a single layer has been found to substitute for the previous diffusion barrier and nucleation layers.
It is an object of the present invention to provide superconducting films, especially YBCO superconducting films, on polycrystalline substrates such resultant articles demonstrating properties such as high Jc's and Ic's.
It is another object of the present invention to provide structural template articles for subsequent deposition of oriented films, e.g., superconducting films, especially YBCO superconducting films.